Researchers from Southwest Research Institute (SwRI) and Eaton have demonstrated the application of downspeeding and supercharging a diesel vehicle, in conjunction with optimizing transmission gear ratios and the shift schedule, to deliver a greater improvement in fuel economy than a downsped and turbocharged configuration.

The shift schedule optimization technique resulted in fuel economy gains of up to 12.2% along with a reduction in shift frequency of up to 55% relative to a baseline turbocharged configuration. A downsped turbocharged configuration showed up to a maximum 6.6% improvement over the baseline. The downsped supercharged configuration also retained the first gear acceleration, top gear passing, and 0-60 mph acceleration of the baseline turbocharged vehicle. The team presented its work at the SAE 2012 World Congress.

Engine downsizing and downspeeding are two strategies that can be applied to gasoline and diesel engines to improve vehicle fuel economy. Forced induction is a necessary aspect of these strategies in order to maintain vehicle performance for customer acceptance. The primary methods of forced induction are turbocharging and supercharging, and the choice between these methods depends on the requirements for packaging, torque rise rate, fuel economy, and cost...Turbocharging tends to have an advantage in engine fuel consumption, whereas supercharging tends to have an advantage in torque rise rate, especially at lower engine speeds.

It is possible to leverage the greater low-speed torque rise rate afforded by the supercharger to enable a higher degree of downspeeding while maintaining vehicle performance metrics by optimizing the transmission shift schedule.

—Ostrowski et al.

In the study described in the paper—which was a continuation of previous work on pursuing fuel economy improvements via downspeeding—the team used a turbocharged 2.0L inline four-cylinder engine with a development-level Tier 2 Bin 5 calibration. The engine had a dual loop EGR system to provide high levels of EGR and correspondingly low levels of engine-out NOx.

The team first tested the base turbocharged engine. The researchers then removed the variable speed turbocharger and replaced it with an Eaton R570 supercharger with TVS Supercharger technology. The supercharger was sized and geared to match the full load torque curve of the baseline engine, and was belt-driven by the engine at a speed ratio of 4.3:1.

A bypass valve controlled boost pressure, and the throttle was moved upstream of the supercharger to minimize the supercharger power consumption during throttled operation.

They adjusted the engine calibration to provide similar air-fuel ratio and EGR targets to the baseline configuration; this resulted in a less than 1% difference in engine-out NOx. BSFC (brake specific fuel consumption) gradually increased with supercharging, up to a maximum of 12% at high load. Torque rise was faster with the supercharged configuration, especially at low engine speed; supercharging delivered reductions of up to 77% in time-to-torque.

Once the fuel consumption and torque response of both turbocharged and supercharged versions were measured, the team began working with transmission gear ratios and shift schedule optimization. They performed fuel economy and 0-60 mpg acceleration simulations for three shift schedule optimization strategies for 81 transmission gear ratio configurations of the supercharged configuration. The also created revised shift schedules for the baseline gear ratios for the turbo for comparison.

The downspeeding analysis showed that fuel economy could be improved with supercharging while retaining the baseline vehicle first gear grade-ability, top gear passing capability, and 0-60 mph acceleration time while adhering to industry-standard constant vehicle speed progression gear ratios. The hardware configuration paired with a balanced shift schedule optimization strategy increased the engine load sufficiently to more than overcome the increased specific fuel consumption inherent with the supercharged configuration. As a result, the downsped supercharged powertrain configuration was able to achieve better fuel economy than both the stock and downsped turbocharged powertrain configurations for this vehicle application.

@A D,
What evidence do you have to declare that the experiment in this article never took place?

It is quite believable that a highly-advanced, highly-twisted Roots-type supercharger by Eaton can produce a lot of boost at low engine speeds without the typical turbo lag and low turbocharger's boost at low engine speed. The immediately-available high boost at low rpm minimizes gear shift and keeps the engine in the higher efficient point in the map more often.

During throttle operation at cruise, very little power is consumed by the EAton supercharger, and the benefit in efficiency gain came from downsizing.
A turbocharged engine exhibit similar effect during cruise, during which timne, the turbocharger contributes almost nothing to power nor efficiency, and the efficiency gain came mostly from engine downsizing. However, a supercharged engine with this EAton TVS technology delivers much better transient response at low engine rpm and is thus overall more efficient.

When diesels are considered in conjunction with multiratio transmissions it should borne in mind that there is a very restricted RPM range here with diesel fuel as compared to gasoline.

Whereas modern gasoline engine design is good to nearly 7000rpm with constant torque, its diesel equivalent is only good to 2400rpm while its torque curve is firmly bent over with its shorts around its ankles by 4200rpm. Expect to see a 33% roll off and that's with turbocharging I might add.

The perseverance to persuade what is almost a constant speed prime mover to be useful in a variable speed application - which is what a road vehicle is all about - never fails to surprise me. Hey fellas this is 2012 not 1950.

I would've thought that 2 million Prii have more than proved that decoupling the engine from the wheels is the way to achieve the ultimate in downsizing. In most circles it has been acknowledged that the 1.5L Prius powerplant is more than able to match the performance of a more conventionally coupled 2.5L Camry engine.

Good point, T2.
However,driving is more than just a means to get from point A to point B. It is about fun and self expression. Many people prefer the neck-snapping of high-power acceleration when downshifting using a manual transmission. Now you have the extra boost from a high-quality Eaton supercharger AND the superior fuel efficiency. Now you can have it all.

For a motoring enthusiast, there is no transmission that can beat the fun of a manual transmission.

Hydraulic hybrid operation can match efficient engine speeds with vehicle speeds. And it can produce neck stretching acceleration, but what people want is to waste a lot of fuel to make a lot of noise to move a large engine through very high air friction loss travel at the highest permitted speeds and a bit over. Perhaps there should also be a "green_driver_congress". ..HG..